weiser-etal_2007_some computer science issues in ubiquitous computing
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8/8/2019 Weiser-Etal_2007_Some Computer Science Issues in Ubiquitous Computing
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S Q M E O M PU T RS C | E N G G I S S U E S I N
m
U b i q u i t o u s c o m p u t i n g e n h a n c e s c o m p u t e r u s e b y
m a k i n g m a n y c o m p u t e r s a v a il a bl e t h r o u g h o u t t h e
p h y s i c a l e n v i r o n m e n t , w h i l e m a k i n g t h e m e f fe c t iv e l y i n v is i b le
t o t h e u s e r. T h i s a r t ic l e e x p l a i n s w h a t i s n e w a n d d i f f e r e n t
a b o u t t h e c o m p u t e r s c ie n c e in v o lv e d in u b i q u i t o u s c o m p u t i n g .
F i rs t, i t p r o v i d e s a b r i e f o v e r v i e w o f u b i q u i t o u s c o m p u t i n g ,
t h e n e l a b o r a t e s t h r o u g h a s e ri e s o f e x a m p l e s d r a w n f r o m
v a r i o u s s u b d i s c ip l i n e s o f c o m p u t e r s c i en c e : h a r d w a r e c o m -
p o n e n t s ( e .g . , c h i p s ) , n e t w o r k p r o t o c o l s , i n t e r a c t i o n s u b s t r a t e s
( e . g . , s o f t w a r e f o r s c r e e n s a n d p e n s ) , a p p l i c a t i o n s , p r i v a c y ,
a n d c o m p u t a t i o n a l m e t h o d s . U b i q u i t o u s c o m p u t i n g o ff er s a
f r a m e w o r k f o r n e w a n d e x c i t i n g r e s e a r c h a c r o s s t h e s p e c t r u m
o f c o m p u t e r s c ie n c e.
S i n c e w e s t a r t e d t h i s w o r k a t X e r o x P a l o A l t o R e s e a r c h
C e n t e r ( P A R C ) i n 1 98 8 a f e w p l a c e s h a v e b e g u n w o r k o n t h is
p o s s i b l e n e x t - g e n e r a t i o n c o m p u t i n g e n v i r o n m e n t i n w h i c h
e a c h p e r s o n is c o n ti n u a l ly i n t e r a c t i n g w i t h h u n d r e d s o f n e a r b yw i r e l es s ly i n t e r c o n n e c t e d c o m p u t e r s . T h e g o a l i s t o a c h i e v e t h e
m o s t e f f e c t iv e k i n d o f t e c h n o l o g y , t h a t w h i c h i s e s s e n t i a l l y
i n v i s i b l e t o t h e u s e r . T o b r i n g c o m p u t e r s t o t h i s p o i n t w h i l e
r e t a i n i n g t h e i r p o w e r w i l l r e q u i r e r a d i c a l l y n e w k i n d s o f c o m -
p u t e r s o f a l l s iz e s a n d s h a p e s t o b e a v a i l a b l e to e a c h p e r s o n . I
c al l t h is f u t u r e w o r l d " U b i q u i t o u s C o m p u t i n g " ( U b i c o m p )
[ 27 ]. T h e r e s e a r c h m e t h o d f o r u b i q u i t o u s c o m p u t i n g i s s t a n -
d a r d e x p e r i m e n t a l c o m p u t e r s c i e n c e : t h e c o n s t r u c t i o n o f
w o r k i n g p r o t o t y p e s o f t h e n e c e ss a i- y i n f r a s t r u c t u r e i n s u f f i c ie n t
q u a n t i t y t o d e b u g t h e v i a b i li t y o f th e s y s t e m s i n e v e r y d a y u s e ;
o u r s e l v e s a n d a f e w c o l l e a g u e s s e r v i n g a s g u i n e a p i g s . T h i s i s
Reprinted with permission. Copyright c) 1993 by The A ssociation orComputing Machinery, n c. All rights reserved. This article originallyap-peared in Communications f the ACM , Vol. 36, No. 7 (July 1993), Pages75-84.
12 Mobile Computing and Communications Review, Volume 3, Number 3
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a n i m p o r t a n t s t e p t u w a r d e n ~ t a r i n g
t h a t o u r i n f i ' a s t r u c t u r e r e s e a r c h i s
r o b u s t a n d s c a l a b l e i ~ t h e f a c e o f t h e
de ta i l s o f the rea l wor ld .
" I' he i d e a o f u b i q u i t o u s c o m p u t i n g
f i r s t a r o s e t : ¥ o m c o n t e m p l a t i n g t h e
p l a c e o f t o d a y ' s c o m p u t e r i n a c t u a l
a c t i vi t i es o f e v e r y d a y l i f e , I n p a r t i c u -
l a r , a n t h r o p o l o g i c a l s t u d i e s o f w o r k
l i f e [14 , 22 ] teach u s tha t peop le p r i -
m a r i l y w o r k i n a w o r l d o f s h a r e d s i t-
u a t i o ns a n d u n e x a m i n e d t e ch n o l og i -
c a l s k i l l s . T h e c o m p u u e r t o d a y i s
i s o l a t e d f r o m t h e o v e r a l l s i t u a t i o n .
h o w e v e r , a n d f a i l s t o g e t o u t o f t h e
w a y o f t h e w o r k . I n o t h e r w o r d s ,
r a t h e r t h a n b e i n g a t o o l t h r o u g h
w h i c h w e w o r k , a n d t h u s d i s a p p e a r -
i n g f r o m o u r a w a r e n e s s , t h e c o m -
p u t e r t o o o f t e n r e m a i n s t h e g o c us o f
a t t e n t i o n . A n d t h i s is t r u e t h r o u g h -
o u t t h e d o m a i n o f p e r s o n a l c o m p u t -
i n g as c u r r e n t l y i m p l e m e n t e d a n d
d i s c u s se d i b r t h e f u t u r e , w h e t h e r
o n e t h i n k s o f p e r s o n a l c o m p u t e r s .p a h n t o p s , o r d y n a N ) o k s , T b e c h a r a c-
t e r i z a t i o n o f t h e f u t u r e c o m p u t e r a s
t h e " i n t i m a t e c o m p u t e r " [ t2 1 , o r
" r a t h e r l i k e a h u m a n a s s i s t a n t " [ 2 5 1
m a k e s t h i s a t t e n t i o n t o t h e m a c h i n e
i t se l f p a r t i c u la r l y a p p a r e n t .
G e t t i n g t h e c o m p u t e r o u t o f t h e
w a y i s n o t e a s y . T h i s i s n o t a g r a p h i -
c a l u s e r i n t e r f a c e ( G U I) p r o b l e m , b u t
i s a p r o p e r t y o f t h e w h o l e c o n t e x t o f
u s a g e o f t h e m a c h i n e a n d t h e a t t r i -
b u t e s o f i t s p h y s i c a l p r o p e r t i e s : t h e
k e y b o a r d , t h e w e i g h t a n d d e s k t o p
p o s i t i o n o f s c r e e n s , a n d s o o n . T h e
p r o b l e m i s n o t o n e o f " i n t e r f a c e . "F o r t i l e s a m e r e a s o n o f c o n t e x t , t h i s
i s n o t a m u l t i m e d i a p r o b l e m , r e s u l t -
i n g f r o m a n y p a r t i c u l a r d e f i c i e n c y i n
t h e a b i l i t y t o d i s p l a y c e r t a i n k i n d s o f
r e a l - t i m e d a t a o r i n t e g r a t e t h e m i n t o
a p p l i c a t i o n s . ( I n d e e d , m u l t i m e d i a
t r i e s t o g r a b a t t e n t i o n , t h e o p p o s i t e
o f t h e u b i q u i t o u s c o m p u t i n g i d ea l o f
i n v i s i b i li t y . ) T h e c h a l l e n g e i s t o c r e -
a t e a n e w k i n d o f r e l a t i o n s h i p o f p e o -
p l e t o c o m p u t e r s , o n e i n w h i c h t h e
c o m p u t e r w o u l d h a v e t o t a k e t h e
l e a d i n b e c o m i n g v a s tl y b e t t e r a t g e t -
t i n g o u t o f t h e w a y , a l l o w i n g p e o p l e
t o j u s t g o a b o u t d l e i r l iv e s .I n [ 9 8 8 , w h e n I s t a r t e d P A R C ' s
w o r k o n u b i q u i to u s c o m p u t i n g , v i r -
t u a l r e a l i t y ( V R ) c a m e t h e c l o s e s t t o
e n a c t i n g t h e p r i n c i p l e s w e b e l i e v e d
i m p o r t a n t . I n i t s u l t i m a t e e n v i s i o n -
m e n t , V R c a u s e s t h e c o m p u t e r t o
: | | J
b e c o m e e f f e c t i v e l y i n v i s ib l e by i n k i n g
o v e r t he h u m a n s e n s o r y a n d a i t e c to r
s y s t e m s [ 1 9 ] . V R i s e x t r e m e l y u s e f u l
i n s c i e n t i f i c v i s u a l i z a t i o n a n d e n t e r -
t a i n m e n t , a n d w i ll b e v e r y s i g n i f i c a n t
f i.~r those n iches . B u t a s a too l fo r
p r o d u c t i v e l y c h a n g i n g e v e r y o n e ' s
r e l a t i o n s h i p t o c o m p u t a t i o n , i t h a s
two c ruc ia l f l aws : f i r s t , a t the p re sen t
t i m e ( 1 9 9 2 ) , a n d p r o b a b l y f o r d e -c a d e s , it c a n n o t p r o d u c e a s i m u l a t i o n
o f s i g n i f i c a n t v e r i s i m i l i t u d e a t r e a -
s o n a b l e c o s t ( to d a y , a t a n y c o s t) . T h i s
m e a n s t h a t u s e r s w i l l n o t b e t o o l e d
a n d t h e c o m p u t e r w i l l n o t b e o u t o f
t h e w a y . S e c o n d , a n d m o s t i m p o r -
t a n t , i t h a s t h e g o a l o f f o o l i n g t h e
u s e r - - o f l e a v i n g t h e e v e r y d a y p h y s i-
c a l w o r l d b e h i n d . T h i s i s a t o d d s w i t h
t h e g o a l o f b e t t e r i n t e g r a t i n g t h e
c o m p u t e r i n t o h u m a n a c ti v it i es , s i n ce
h u m a n s a r e o f a n d i n t i l e e v e r y d a y
w o r l d .
U b i q u i t o u s c o m p u t i n g i s e x p l o f
i n g q u i t e d i f f e r e n t g r o u n d f r o m p e r -s o n a l d i g i t a l a s s i s t a n t s , o r t h e i d e a
t h a t c o m p u t e r s s h o u l d b e a u t o n o -
m o u s a g e n t s t h a t t a k e o n o u r g o a l s .
T h e d i f f e r e n c e c a n b e c h a r a c t e r i z e d
a s f o l lo w s : S u p p o s e y o u w a n t t o l i f t a
h e a v y o b j e c t . Y o u c a n c a l l i n y o u r
s t r o n g a s s i s t a n t t o l i f t i t t o t y o u , o r
y o u y o u r s e l f c a n b e m a d e e f f o rt -
l e s s l y , u n c o n s c i o u s l y , s t r o n g e r a n d
j u s t l i f t i t. T h e r e a r e t i m e s w h e n b o t h
a r e g o o d . M u c h o f t h e p a s t a n d c u r -
r e n t e f f o r t f b r [ l e t t e r c o m p u t e r s h a s
b e e n a i m e d a t th e f o r m e r ; u b i q u i t o u s
c o m p u t i n g a i m s a t t h e l a t t e r .
T h e a p p r o a c h I t o o k w a s t o a t -t e m p t t h e d e f i n i t i o n a n d c o n s t r u c -
t i o n o f n e w c o m p u t i n g a r t i f a c t s f o r
u s e i n e v e r y d a y l i f e. I t o o k m y i n s p i -
r a t i o n f r o m t h e e v e r y d a y o b j e c t s
f o u n d i n o f f i c e s a n d h o m e s , i n p a r -
t i t u l a r t h o s e o b j e c t s w h o s e p u r p o s e i s
t o c a p t u r e o r c o n v e y i n f o r m a t i o n .
T h e m o s t u b i qu i t o u s c u r r e n t i n t o r -
m a t i o n a l t e c h n o l o g y e m b o d i e d i n
a r t i f a c t s i s t h e u s e o f w r i t t e n s y m b o l s ,
p r i m a r i l y w o r d s , b u t i n c l u d i n g a ls o
p i c t o g r a p h s , c l o c k s , a n d o t h e r s o r t s
o f s y m b o l i c c o m m u n i c a t i o n . R a t h e r
t h a n a t t e m p t i n g t o r e p r o d u c e t h e s e
o b j e c t s i n s i d e t h e v i r t u a l c o m p u t e r : t
w o r l d , l e a d i n g t o a n o t h e r " d e s k t o p
m o d e l " [ 2 1 , I w a n t e d t o p u t t h e n e w
k i n d o f c o m p u t e r o u t i n i b i s w o r l d o f
c o n c r e t e i n f o r m a t i o n c o n v e y e r s .
S i n c e t h e s e w r i t t e n a r t i f a c t s o c c u r i n
m a n y d i f t e r e n t s i z es a n d s h a p e s , w i th
m a n y d i f f i e r e n t q u a l i t ie s , 1 w a n t e d
t h e c o m p u t e r e r n b o d i m e n t s u ) be o f
m a n y s iz es a n d s h a p e s , i n c l u d i n g t in y
i n e x p e n s i v e o n e s t h a t c o u l d b r i n gc o m p u t i n g t o e v e r y o n e .
T h e p h y s k a l w o r l d c o m e s i n a l l
s i z e s a n d s h a p e s . F o r p r a c t i c a l r e a -
s o ns o u r u b i q u i to u s c o m p u t i n g w o r k
t ~ g i n s w i t h j u s t t h r e e d i f f e r e n t s i z es
o f d e v ic e s : e n o u g h t o g i ve s o m e
s c op e , n o t e n o u g h t o d e t e r p r o g r e s s.
T he f i r s t s ize i s the wa ll -s~zed in t e rac -
t i v e s u r f h c e , a n a l o g o u s t o t h e o f f i c e
w h i t e b o a r d o r t h e h o m e m a g n e t -
c o v e r e d r e f r i g e r a t o r o r b u l l e t i n
t x ) a r d . T h e s e c o n d s i z e i s t h e n o t e -
p a d , e n v i s i o n e d n o t a s a p e r s o n a l
c o m p u t e r b u t a s a n a l o g o u s t o s c r a p
p a p e r t o b e g r a b b e d a n d u s e d e a s il y ,w i t h m a n y b e i n g u s e d b y a p e r s o n a t
o n e t i m e . T h e c l u t t e r e d o f f i c e d e s k
o r m e s s y t i ' o n t - h a l l t a b l e a r e r e a l - l i f e
e x a m p l e s . F i n a l l y , t h e t h i r d s i ze i s t h e
t i n y c o m p u t e r , a n a l o g o u s t o t i n y in -
d i v i d u a l n o t e s o r P o s t - i t n o t e s , a n d
a l s o s i m i l a r t o t h e t i n y l i tt l e d i s p l a y s
o f w o r d s f o u n d o n b o o k s p i n e s, l i g ht
s w i t c h e s , a n d h a l l w a y s . A g a i n , I s a w
t h i s n o t a s a p e r s o n a l c o m p u t e r , b u t .
a s a p e r v a s i v e p a r t o f e v e r y d a y l i f e ,
w i t h m a n y a c t i v e a t a ll ti m e s ; I c a l l e d
t h e s e t h r e e s i z e s o f c o m p u t e r s
I ~ ) a r d s , p a d s , a n d t a b s , a n d a d o p t e d
t h e s l o g a n t h a t , f o r e a c h p e r s o n i n a no f f i c e , t h e r e s h o u l d b e h u n d r e d s o f
t a b s , t e n s o f ' p a d s , a n d o n e o r t w o
b o a r d s . S p e c i f i c a t i o n s f o r s o m e p r o -
t o t y p e s o f t h e s e t h r e e s i z e s i n u s e a t
P A R C a r e s h o w n i n F i g u r e 1,
T h i s t h e n i s P ha s e I o f u b i q u i t o u s
c o m p u t i n g : t o c o n s t r u c t , d e p lo y , a n d
7G July 1~93!Va136, Nod ~, lm ~A 't "~ I : ~ l T J , m ~ , ~ m
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learn from a COl n p u t i r i g environ-
ment con sisting of tabs, pads, and
hoards. This is only Phase I, because
it is unlikely to achieve optimal invisb
bility. (Later phases are yet io be de-
termined.) tint it is a start down the
radical direction, for computer sci-
erlce, away from emphasis on the
macliine and back to the person and
his or laer life in the world of' work,
play, and home.
Hardware PrototypesNew hardware systems design tot
ubiquitous computing has been ori-
ented toward experimental plat-
forms for systems and ap plications of
invisibility. New chips have been less
important than combinations of ex-
isting contp onent s that create experi-
ntental opportunities. The first ubiq-
uitous computing technology to be
deployed was the Liveboard [6],
which is now a Xerox product, Two
otlter important pieces of prototype
hardware supportin g our research at
PAR(; are the Tab and the Pad.
Tats
The Parc'[hb is a tiny information
doorway. Fox user intera ction it has a
pressure-sensitive screen on top of
the display, three buttons positioned
underneath the natural finger post-
lions, and the ability to sense its posi-
tion within a building. The display
and touchpad it uses are standard
commercial units.
The key hardware design prob-
lems atfecting the pad are physicalsize and power consur nption. With
several dozens of these devices sitting
around the oflice, in briefi.'ases, in
pockets, one cannot change their bat-
|cries every week. The PARC design
uses the 8051 chip to control detailed
interactions, and includes soliware
that keeps power usage down. The
major outboard components arm a
srnall analog/digital converte r fo r the
pressure-sensitive screen, and analog
sense circuitry for the IR receiver.
Interestingly , aldaough we have been
approached by several chip manu-
facturers ab out our possiMe ne ed for
custo m chips t~)r the Ta b, the "l~tb is
not short of places to put chips. The
display size leaves plenty of room,
arid the display thickness dominates
total size. Off-the-shelf componentsare more than adequate for explor-
ing this design space, even with our
severe size, weight, and power con-
strain|s.
A key part of our design philoso-
phy is to put devices in everyday use,
not just demonstrate them. We can
only use techniques suitable for
quantity 100 replication. This ex-
cludes certain tedmiques that could
make a hugs difference, such as the
integration of components onto t h e
display surlace itself'. This technol-
ogy is being explored at PARC, aswell as other research organizations.
While it is very pr omis ing, it is not yet
ready ff)r replication.
"fire Tab architecture incorporates
a careful balance of' display size,
bandwidth, processing, and memory.
For instance , the small display means
that even the tiny processor is capa-
ble ot providing a four-frames-per-
second video rate, and the IR band-
width is capable of delivering this.
The bandwidth is also such that the
preKessor can actually time the pulse
widths in software timing loops. (.)ur
current design has insufticient stof
age, and we are increasing ~he
amount of nonvolatile RAM in fk~-
ture tabs from 8K to 128K. The tab's
goal of casual use similar to that oi
Post-it notes puts it into a design
space generally unexplored in the
commercial or research sector.
P a d
The pad is really a family of note-
t~ok-sized devices. Our initial pad,
the ScratchPad, plugged into a Sun
SBus card and provided an
X-Window-sysmm-com atible writ-
ing and display surt~ce. This same
design was used inside our first wall-
sized displays, the liveboards, as well.
Our later untethered pad devices,
the XPad and MPad, continued the
system design principles of
X-compatibility, ease of con struct ion,
and flexibility in software and hard-ware expansion.
A s l write this article, at the e n d o f
1992, commercial portable pen de-
vices have linen on the market fbr
two years, altltough most of the early
companies have now gone out of
business. Why should a pioneering
research lab build its own such de-
vice? Each year we ask ourselves the
sarne question, and so tar three
things always drive us to continue to
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design om ~)wn pad hardware:
First, we need the correct balance
of ti:atures--this is the essence ot
systems design. 1he commercial de-
,ices all aim at particular niches, bal.
a~lcing their design to that niche. For
research we need a rather difti:rent
balance, particularly tbr ubiquitous
coinputmg, For instance, can tile
device communicate simuttaaeouslyalong multiple chaimets? Does the
operating system support muttipro-
cessing? What about the potential for
high-speed tethering? ts there a
high-quality pen? Is there a high-
speed expansion port sufficient for
video in and out? Is sound in/out and
ISDN commctivity available? Op-
tional keyboard? Any one commer-
cial device tends to satist}' some of
these needs, ignore other.s, and
choose a balance of the ones it does
satisfy, optimizing its niche, rather
than ubiquitous computing-style
scrap computing. The balance [or us
emphasizes communication, system
memory, muhimedia, and expansion
ports.
Second, apart from b alance of flea-
rares are the requirements for par-
ticular features. Key among these are
a pen emphasis, connection to re-
search envi ronm ents such as Unix,
and colnnmnication emphasis. A
high-speed (>64KB/sec) wireless
capability is not built into any com-
mercial devices, and they do not gen-
erally have a sufficiently high-speed
port to add such a radio. Conmmrcialdevices generaUy tonre with DOS or
Penpoint, and while we have devel-
oped in both, they are not our filvor-
ire research vehicles because they
lack full access and customizability.
The third factor driving out own
pad designs is ease of expansion and
modification. We need flail hardw are
specifications, complete operating
system sotuce code, and the ability to
remove and replace both hardware
and software components. Naturally
these goals are oppose d to best price
in a niche market, which orients the
documentation to the end user, andkeeps prices down by integrated
rather that) modular design.
We have built and used three gen-
erations of Pad designs. Six scratch-
pads were built, three XPads, and 13
MPads, the latest. The MPad uses an
FPGA for almost all random logic,
giving extreme flexibility. For in-
stance, changing tile power comrol
ftulctious and adding higtl-quatity
sound were relatively simple FPGA
changes. The MPad has both IR (tab
compatible) and radio communica-
tion built-in and indudes sufficient
uncommitted space tor adding new
circuit boards later. It can be used
with a tether that provides it withrecharging and operating power and
an Ethernet connection. The operat-
ing system is a st andalone version of
the public-do main Portable Common
Runtime developed at PAR('. [28].
1"he C o m pute r Sc ie nce o fU b l c o m plib construct and deploy tabs, pads,
and boards at PAR(:, we found our-
selves having to readdress some of
the well-worked areas of existing
computer science. The fluitfulness
of ubiquitous computing [br new
computer science problems justified
out belief in the ubiquitous comput-
ing framework.
The ibllowing subsections "as-
cend" the levels of organization of a
computer system, from hardware to
application. One or two examples of
computer science work required by
ubiquitous computin g are described
hn- each level. Ubicomp is not yet a
coherent body of work, but consists
of t few scattered communities. The
point of this article is to help others
understand some of the new re-
search challenges in ubiquitous com-puting, and inspire them to work on
them. This is more akin to a tutorial
than a survey, and necessarily selec-
tive, The areas included are hard-
ware components (e.g., chips), net-
work protoc,)ls, interaction sub-
strates (e.g., software for screens and
pens), applications, privacy, anti
computational methods.
I ss ue s o f H a rc lwa reC o m p o n e n t sI n a d d i t i o n t o t h e n e w s y s t e m s o f
t a b s , p a d s , a n d b o a r d s , u b i q u i t o u s
c o m p u t i n g n e c e s s i t a t e s s o m e n e wk i n d s o f d e v i c e s . E x a m p l e s o f t h r e e
n e w k i n d s o f h a r d w a r e d e v i c e s a r e
v e r y l o w - p o w e r c o m p u t i n g , l o w -
pow,e r h i g h - b i t s / c u b i c - m e t e r c o m m u -
n i c a t i o n , a n d p e n d e v i c e s .
LOW Pow erI n g e n e r a l t h e n e e d f o r h ig h p e r f o r-
mance has dominated the need t0r
low-power consm npti on in processor
design. However, r ecognizing the
new requirements of ubiquitous
computing, a numbe r of people have
begun work in using additional chip
area to reduce power rather than to
increase performance [16]. One key
approach is to reduce the clocking
frequeucy of their chips by increas-ing pipelining or parallelism. Then,
hy ru,min g the chips at reduc ed volt-
age, the effect is a net reduction in
power, because power falls off as the
square of the voltage, while only
about twice the area is neede d to ru n
at haft the clock speed.
Power = Cl*Vdd'~*]"
where Gt. is the gate capacitance,
Vda the supply voltage,
and f the clocking frequency.
This method of reducing power
leads to two new areas of chip design:
circuits that will run at low power,
and architectures that sacrifice area
lbr power over performance. The
second requires some additional
comment, because one might sup-
pose one would simply design the
fastest possible chip, and then run it
at reduced clock and voltage. How-
ever, as Lyon illustrates, circuits in
chips designed for high s p e e d g e n e r -
a l ly fail to work at low voltages. Fur-
thermore, attent ion to special circuits
may permit operation over a much
wider range o f vohage operation, or
achieve power savings via other spe-cial techniques, such as adiabatic
switching [ 16].
WirelessA w i r e le s s n e t w o r k c a p a b l e o f a c c o m -
m o d a t i n g h u n d r e d s o f h i g h - s p e e d
d e v i c e s f o r e v e r y p e r s o n i s w e l l b e -
y o n d t h e c o m m e r c i a l w i r e l e s s s y s -
t e m s p l a n n e d f o r t h e n e x t 1 0 y e a r s
[ 2 0] , w h ic h a r e a i m e d a t o n e l o w -
s p e e d ( 6 4 k b / s e c o r v o i c e ) d e v i c e p e t
p e r s o n . M o s t w i r e l e s s w o r k u s e s a
f i g u r e o f m e r i t o f b i t s /s e c x r a n g e ,
a n d s e e k s t ( ) i n c r e a s e t h i s p r o d u c t .
W e b e l i e v e t h a t a b e t t e r f i g u r e o tm e r i t i s b i t s / s e c / m e t e r ~ . T h i s f i g u r e
o f m e r i t c a u s e s t h e o p t i m i z a t i o n o f
t o t a l b a n d w i d t h t h r o u g h o u t a 3 D
s p a c e , l e a d i n g t o d e s i g n p o i n t s o f
v e r y t i n y c e l l u l a r s y s t e m s ,
B e c a u s e w e f e l t t h e c o m m e r c i a l
w o r l d w a s ig n o r i n g t h e p r o p e r f i g u r e
~ 8 J u i ~, 1 9 ~ .~ 3 /\ '~ L 3 6 , N o 7 C O ~ 4 M U l U C 4 ~ q O t 4 U s o l , ~ r m t a c ~ a
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, + k , ] o > i . L o ,
T comtruct and dephy tab~, pad5, and board5 at PA RC~ weJbuT zd oursdves
hav ing to readdress som e of the well-w orke d areas of existing co mp uter science.
of merit, we initiated our own small
radio program. In 1989 we built
spread-spectrum transceivers at900MHz, but fbund them difficult to
build and adjust, and pr one to noise
and multipath interference. In 1990
we built direct frequency-shift-keyed
transceivers also at 900MHz, using
very low power to be license-fi'ee.
While much simpler, these transceiv-
ers had unexpectedly and unpredict-
ably long range, causing mutual in-
terference and multipath problems.
In 1991 we designed and built our
current radios, which use the near-
field of" the electromagnetic spec-
trum. The near-field has an effective
fall-off of r 6 in power, instea d of t he
more u sual r 2, where r is the distance
from the transmitter. At the proper
levels this band does not require an
FCC license, permits reuse of the
same frequency over and over again
in a building, has virtually no muhi-
path or blocking effects, and permits
transceivers that use extremely low
power and low parts count. We have
deployed a number of near-field
radios within PARC.
P e n s
A third new hardware component isthe pen f2)r very large displays. We
needed pens that would work over a
large area (at least 60in x 40in), not
require a tether, and work with back
projection, "l'hese requirements are
generated from the particular needs
of large displays in ubiquitous com-
put ing -ca sua l use, no training, nat-
uralness, simultaneous muhiple use.
No existing pens or touchpads could
come close to these requirements.
Therefore members of the Electron-
its and Imag ing lab at PARC devised
a new infrared pen. A camera-like
device behind the screen senses thepen position, and information about
the p en state (e.g., but tons) is modu-
lated along the IR beam. The pens
need not touch the screen, but can
operat e f rom several feet away. Con-
siderable DSP and analog design
work underlies making these pens
effective components of the ubiqui-
tous coruputmg system [6].
N e t w o r k P r o t o c o l sUbicomp changes the emphasis m
networking ira at least four areas:
wireless media access, wide-band-
width range, real-time capabilities
for multimedia over- standard net-
works, and packet routing.
Wire less Media Access
A "media access" protocol provides
access to a physical medium. Com-
mon media access methods in wireddomains are collision detection and
token-passing. Thes e do n ot work
unchanged in a wireless domain be-
cause not every device is assured of
being able to hear every othe r device
(this is called the "hidden terminal"
problem). Furt her more , earlier- wire-
less work used assumptions of com-
plete autonomy, or a statically con-
figured network, while ubiquitous
computing requires a cellular topoi-
ogy, with mobile devices frequently
comin g on- and of f line. We have
adapted a media access protocol
called MACA, first described by
Karn [11], with some of our own
modifications for fairness arid eft]-
ciency.The key idea of MACA is for tire
two stations desiring to communica te
to first do a short handshake of Re-
quest-To-Send-N-bytes followed by
Clear-'lb-Send-N-bytes. This ex-
change allows all other stations to
hear that there is going to be traffic,
and for how long they should remain
quiet. Collisions, which are detected
by timeouts, occur only during the
short Request-To-Send packet.
Adapting MACA for ubiquitous
computi ng use required considerable
attention to fairness and real-time
requirements. MACA requires sta-tions whose packets collide to back
off" a ra ndo m time an d try again. If
all stations but one back-off, that one
can dominate the bandwidth. By re-
quir ing all stations m a dapt the back-
off para meter of their neighbors, we
create a much tairer allocation of
bandwidth.
Some applications l~¢.ed guaran-
teed bandwidth for voice or video.
We added a new packet type,NCTS(n) (Not Clear *lk) Send), to
suppress all other transmissions for
(n) bytes. This packet is sufticient for
a basestation to do effective band-
width allocation among its mobile
units. "l'he solution is robust, in the
sense that if the basestation stops
allocating bandwidth the system
reverts to norIIlill COIItCClltiOn.
When a number of mobile units
share a single basestation, that base-
station may be a bottleneck for com-
munication. For fhirness, a basesta-
tion with N > 1 nonemp ty ou tput
queues needs to contend for band-
width as though it were N stations.
We therefore make the basestatkm
cont end .just eno ugh more aggres-
sively that it is N times mor e likely to
win a contention tor media access.
O t h e r N e t w o r k I S S U E S
* [M:-o other areas of networking re-
search at PARC with ubiquitou s com-
puting implications are Gh networks
and real-time protocols. Gb-per.
second speeds are important because
of the increasing number of me-
dium-speed devices anticipated byubiquitous computing, and the grow-
ing importance of real-time (muhi-
media) data. One hun dre d 256kb/sec
portables p er office implies a Gb per*
grou p of 40 offices, with all of PARC
needin g an aggregate of some five
Gb/sec. This has led us to do research
into local-area NI'M switches, in as-
sociation with other Gb networking
projects [151.
Real-time protocols are a new area
of h)cus in packet-switched networks.
Although real-time delivery has al-
ways been important in telephony, a
few h undr ed milliseconds never mat-tered in typical packet-switched ap-
plicatinns such as telnet a nd file
transfer. With the ubiquitous use of
packet-switching, even ['or teleph ony
using ATM, the need for real-time
capable protocols has become urgent
if the packet networks are going to
support muhimedia applications.
16 Mobile Computing and Communications Review, Volume 3, Number 3
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Again, in associatio~ with other
members of the research community,
PARC is exploring new protocols tb~
enabling multimedia on the packet-
swkched Internet [4].
The Inter net routing protocol, IP,
has been in use for over 10 years.
However, neither this protocol nor
its OSI equivalent, CLNP, providessufficient iniYastructure 1or highly
mobile devices. Both interpret fields
in the network names of devices m
order to route packets to the device.
For instance, the "13" in IP name
13.2.0.45 is interpreted to mean net
13, and network routers anywhere in
the world are expected to know how
to get a packet to net 13, and all de-
vices whose name starts with 13 are
expected to be on that network. This
assum ption fails as soon as a user of a
net t 3 mobile device takes her device
on a visit to net 36 (Stanford).
Changing the device name dynami-cally dep end ing on location is no so-
lution: higher-level protocols such as
TCP assume that underlying names
will not change during the life of a
connection, and a n ame change must
be accompanied by informing the
entire network of the change so that
existing services can fin d the device.
A nu mbe r of solutions have been
proposed to this problem, among
them Virtual IP from Sony [24], and
Mobile IP from Col umbia University
[ 10]. "l'hese solutions p erm it existing
IP networks to interoperate trans-
pare ntly with roam ing hosts. The key
idea of all approaches is to add a sec-
ond layer of IP address, the "real"
address indicating kKadon, to the
existing fixed-device address. Special
routing nodes that forward packets
to the correct real address, and keep
track of where this address is, are
required for all approaches.*
In teract ion S ubstratesUbicomp has led us to explore new
substrates for interaction. Four such
substrates are men tioned here, span-
ning the space from virtual key-boards to protocols for window sys-
tems.
Tabs have a very small interaction
area~too small for a keyboard, too
small even for standard handprint-
* T h e I n t e r n e t c o m m u n i t y h a s a w o r k i n g g r o u pc o n s i d e r i n g s t a n d a r d s f o r t h i s a r e a ( c o n t a c td e e r i n g @ x e r o x . c o m f o r m o r e i n f o r m a t i o n ) .
ing recognition. Handprmting has
the further problem of requiring
looking at what is written. Improve-
ments in voice recognition are no
panacea, because when other people
are present, voice will often be inap-
propriate. As one possible solution,
we developed a method of touch-
printing that uses only a tiny areaand does trot require looking. A
drawback of our method is it re-
quires a new printing alphabet to be
memorized, and reaches only half
the speed of a fast typist [8].
Liveboards have a high intera ction
area, 400 times that of the tab. Using
conventional pull-down or pop-up
menus might require walking across
the room to the appropriate button,
a serious problem. We have devel-
oped methods of location-indepen-
dent interaction by which even coin-
plex interactions can be popped up
at any location. [13].The X-Window system, although
designed for network use, makes it
difficult for windows to move once
instantiate d at a given X server. This
is because the server retains consid-
erable state about individual win-
dows, and does not provide conve-
nient ways to move that state. For
instance, context and window IDs are
dete rmin ed solely by the server, and
cannot be transferred m a new
server, so applications that depend
on knowing their value (almost all)
will break if a window changes serv-
ers. However, in the ubiquitous corn-
puling world a user may move fre-
quently from device to device, and
want to bring windows along.
Christian Jacobi at PAR(; has im-
plemented a new X toolkit that facili-
tates window migration. Applications
need not be aware that they have
moved from one screen to another;
or if desired, the user can be so in-
forme d with an upcall. We have writ-
ten a numb er of applications on top
of this toolkit, all of which can be
"whistled up" over the network to
follow the user from screen toscreen. Th e a uthor, for instance, fre-
quently keeps a single program de-
velopment and editing environment
ope n for days at a time, migrating its
windows back and forth from home
to work and back each day.
A f inal window system problem is
bandwidth. The bandwidth available
to devices in ubiquitous computing
can vary front Kb/sec to Gb/sec, and
with window migration a single ap-
plication may have to dynamically
adjust to bandwidth over time. The
X-Window system protocol was pri-
marily developed for Ethernet
speeds, and most of the applications
written in it were similarly tested at10Mb/sec. To solve the problem of
efficient X-Window use at lower
bandwidth, the X consortium is
sponsoring a "Low Bandwidth X"
(LBX) working group to investigate
new methods of lowering bandwidth.
[7].
ApplicationsApplications are o f course the whole
point of ubiquitous computing. Two
examples of applications are locating
people and shared drawing.
Ubicomp permits the location of
people and objects in an environ-ment. This was first pioneered by
Olivetti Research la bs in Cambridge,
England, in their Active Badge sys-
tem [26]. In ubiquitous comp utin g
we continued to extend this work,
using it for video annotation and
updating dynamic maps. For in-
stance, Figure 2 shows a portion of
CSL early one morning, and the in-
dividual laces are the locations of
people. This map is updated every
few seconds, permitting quick locat-
ing of people, as well as quickly notic-
ing a meeting one might want to go
to (or where one can find a fresh pot
of coffee).
Xerox PARC, EuroPARC, and tile
Olivetti Research Center have built
several different kinds of locauon
servers. Generally these have two
parts: a central database of informa-
tion about location that can be
quickly queried and dumped, and a
group of servers that collect infor ma-
tion about location and update the
database. Inf orma tion about location
can be deduced from logins, or col-
lected directly from an active badge
system. The location database may beorganized to dynamically notify cli-
ents, or simply to facilitate frequent
polling.
Some example uses of location in-
formation are automatic phone for-
warding, locating an individual for a
meeting, and watching general activ-
ity in a bu ildin g to feel in touc h with
8 0 , ] u b 1993/Vot.36.N od C O , I A ~ U m ~ . A ~ O N ' m O ~ T ~
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i t s c y c l e s o f a c t i v i t y ( i m p o r t a m [ () ~
t e l e c o m r n u r in g ).
X e r o x P A R C h a s i n v e s t i g a t e d a
n u m | ) e r o f s h a r e d I I: le et it ~g t o o ls o v e r
t h e p a s t d e c a d e , s t a r t i n g w i t h t h e
( ; o l , a b w o r k [ 2 1 ] , a n d c o n t i n u i n g
w M / v i d e o d r a w a n d c o m m u n e [ 23 1.
T w o n e w t o ol s w e r e d e v e l o p e d for
i n v e s t i g a t i n g p r o b l e m s i n u b i q u i ~ o n s
c o m p u t i n g . T h e f i r s t is ' F i v o l i ( 18 1,
t h e s e c o r t d i s S l a t e , - - e a c h t o o l is
b a s e d o n d i f f e r e n t i m p l e m e t ~ t a t i o n
p a r a d i g m s . F i r s t t h e h ' s im i l a r i t i e s :
b o t h e n ' l p ha s i z e p e n - b a s e d d r a w i n g
o n a s u r f a c e ; b o t h a c c e p t s c a n n e d
i n p u t a n d c a n p r i n t t h e r e s u l t s ; b o t h
c a l l I t ~ l v e s e v e r a l t l s c r s s i ~ l l u | t a l l e -
o u s ly o p e r a t i n g i n d e p e n d e n d y o n
d i f f e r e n t o r t h e s a m e p a g e s ; b o t h
s u p p o r t m u h i p l e p a g e s . T i v o l i h a s a
s o p h i s t i c a t e d x m t i (m o f a s t r o k e a s
s p l in e , a n d h a s a m m l b e r o f f e a t u r e s
m a k i n g u s( : o f p r o c e s s i n g t h e c o n -
t e n t s a r i d r e l a t i o n s h i p s a m o n g
T a b l e 1 . S o m e h a r d w a r e p r o t o t y p e s in us e a t X e r o x P A RC
l.,HIIle IISIOnS: IU~,~- ,~ I*~- ~ ZdlI'CITI
Weight : 7 .2 ozScreen: 6 .2- x 4 .2cm, 128- x 64
m o n o c h r o m e]b u ch i n p u t : p a ss i v e p re ssu re sen s i n gS o u n d : P i ezo sp eak e rWireless in terfaces: IR at 850ran,
DEC/O l i v e tt i a c t iv e b ad g ecompa6ble , 19 .2k baud PWMb aseb an d m o d u l a t i o n , CS MA
Processor: In te l 8051-type, 8k (v l )t28k (v2) nvram
P o r t s : I~ C ex t e rn a l b u s , rech a rg e p o r tBat tery: 12 horn 's cont inuous or est . 2
w eek s n o rm a l u se , rech a rg eab l e
i lu r len sxon s: ZZ.Z- X Z~- X .%/SCgU
Weight : 511bs 4ozScreen: 640- × 480 LCD Display (3
levels of grey)P en : t e t h e red e l ec t ro m ag n e t i c sen s i n gS o u n d : B u i l t - i n m i c ro p h o n e , S p eak e r .
P i ezo B eep e rWi re l e ss In t e r | b ees : 2 5 0 Kb p s Rad i o ,
19.2Khps IRProcesm r: Motorola 68302, 4MB of
DRA M , 1 /2 MB o f V RA M , I /4 MBo f E P R
Ex t e rn a l P o r t s: S t y l u s /m i c ro p h o n e ,PCM CIA , IM B Seria l , RS232, I~C bus,Keylxaard
In t e rn a l P o r t s : S eco n d au d i o ch an n e l ,IS DN , Ex p an s i o n P o r t
B a t t e ry : rech a rg eab l e , 3 h o u rs
Ul[ l l e I lS lOt tS : t ~ 3 1 1 1 , 3XlI | an t i .~L l l f l
W e i g h t : 5 6 01 b ( 2 5 0 k g )S e r e e n : v e r y b r ig h t 4 5 - x 6 5 i n ,
1 0 2 4 - x 7 6 8 m o n o ch ro m epixels, 640- x 480 pixets color,a l so N TS C v i d eo
Pen: IR wirelessS t e reo so u n dN e t w o rk i n g , p ro cesso r , an d p o r t s
d e t e r m i n e d b y c h oi c e o f e m b e d d e dw o rk s t a t i o n , e i t h e r P C o r S u n
12 amps at 115 vol ts
a c t i v i t y f l ' o r nI o c a t o r s
: i
18
¢ O ' M l W U N I C A T I O N I O ~ T H I I ~ J ~ d ) 199~i\~A3b No? 8~
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s l r o k e s . T i v o l i a l s o u s e s g e s t u r e s a s
i n p u t c o n t r o l t o s e l e c t , m o v e , a n d
c h a n g e t h e p r o p e r t i e s o f o b j e c t s o n
t h e s c r e e n . W h e n s e v e r a l p e o p l e u s e
T i v o l i , e a c h m u s t tx : r u n n i n g a s e pa -
r a t e c o p y , a n d c o n n e c t t o t h e o t h e r s .
O n t h e o t h e r h a n d , S l a t e i s c o m -
p l e t e l y p i x e l - b a s e d , s i m p l y d r a w i n g
i n k o n t h e s c r e e n . S l a t e m a n a g e s a l lt h e s h a r e d w i n d o w s f b r a l l p a r t i c i -
p a n t s , a s lo n g a s th e y a r e r u n n i n g a n
X - W i n d o w s e r v e r , s o i t s a g g r e g a t e
r e s o u r c e u s e c a n b e m u c h l o w e r t h a n
T i w ) l i , a n d i t i s e a s i e r t o s e t u p w i t h
l a r g e n u m b e r s o f p a r t i c i p a n ts . I n
p r a c t i c e w e h a v e u s e d s l a t e f r o m a
S u n t o s u p p o r t s h a r e d d r a w i n g w i t h
u s e r s o n M a c s a n d P C s . B o t h S l a te
a n d T i v o l i h a v e r e c e iv e d r e g u l a r u s e
a t P A R C .S h a r e d d r a w i n g t o o l s a r e a t o p i c o f
r e s e a r c h a t m a n y p l a c e s . F o r i n -
s t a n c e , B e l l c o r e h a s a t o o l k i t f b r
b u i l d i n g s h a r e d t o o ls [ 9 ], a n d J a c o b -s e n a t L B L u s e s m u l t i c a s t p a c k e t s t o
r e d u c e b a n d w i d t h d u r i n g s h a r e d
t o ol u se . T h e r e a r e s o m e c o m m e r c i a l
p r o d u c t s [ 3 ] , b u t t h e s e a r e u s u a l l y
n o t m u h i p a g e a n d s o n o t r e a l l y s u i t -
a b l e t o r c r e a t i n g d o c u m e n t s o r i n t e r -
a c t i n g o v e r t h e c o u r s e o f a n e n t i r e
m e e t i n g . T h e o p t i m a l s h a r e d d r a w -
i n g t o o l h a s n o t b e e n b u i l t . F o r i t s
u s e r i n t e r f a c e , t h e r e r e m a i n i s s u e s
s u c h a s m u l t i p l e c u r s o r s o r ( )t ie , g e s -
t u r e s o r n o t , a n d u s i n g a n i n k o r a
c h a r a c t e r r e c o g n i t i o n m o d e l o f p e n
i n p u t . F o r i t s s u b s t r a t e , i s i t b e t t e r t o
h a v e a s i n g l e a p p l i c a t i o n w i t h m u l t i -
p l e w i n d o w s , o r m a n y a p p l i c a t i o n s
i n d e p e n d e n t l y c o n n e c t e d ? I s p a ck e t-
m u l t i c a s t a g o o d s u b s t r a t e t o u s e ?
W h a t w o u l d i t t a k e to s u p p o r t s h a r e d
d r a w i n g a m o n g 5 0 p e o p l e ; 5 , 0 0 0
p e o p l e ? T h e a n s w e r s a r e l i ke l y t o b e
I x> th t e c h n o l o g i c a l a n d s o c i al .
T h r e e n e w k i n d s o f a p p l ic a t io n s o f
u b i q u i t o u s c o m p u t i n g a r e b e g i n n i n g
t o b e e x p l o r e d a t X e r o x P A R C . O n e
i s t o t a k e a d v a n t a g e o f t r u e i n v i s i b i l -
i t y , l i t e r a l l y h i d i n g m a c h i n e s i n t h e
w a l ls . A n e x a m p l e i s t h e R e s p o n s i v e
E n v i r o n m e n t p r o je c t l e d b y S c o ttE h ' o d . " [ 'h i s a i m s t o n t a k e a b u i l d i n g ' s
h e a t , l i g h t , a n d p o w e r m o r e r e s p o n -
s i ve t o i n d i v i d u a l l y c u s t o m i z e d
n e e d s , s av i n g en e r g y a n d m a k i n g a
m o r e c o m t b r t a b l e e n v i r o m n e n t .
A s e c o n d n e w a p p r o a c h i s t o u s e
s o - c a l l e d " v i r t u a l c o m m u n i t i e s " v i a
t h e t e c h n o l o gT o f M U D s . A M U D , o r
" M u h i - U s e r D u n g e o n , " is a p r o g r a m
t h a t a c c e p t s n e t w o r k c o n n e c t i o n s
f r o m m u h i p l e s i m u h a n e o u s u s e r s
a n d p r o v i d e s a c c e s s t o a s h a r e d d a t a -
b a s e o f ' r o o m s ' , ' e x i t s ' , a n d o t h e r o b -
j e c t s . M U D s h a v e e x i s t e d f o r a b o u t
1 0 y e a r s , b e i n g u s e d a h n o s t e x c l u -
s i v e ly t b r r e c r e a t i o n a l p u r p o s e s .
H o w e v e r , t h e s i m p l e t e c h n o l o g y o fM U I ) s s h o u l d a l s o b e u s e f u l i n o t h e r ,
n o n r e c r e a t i o n a l a p p li c a t i o n s, p r o v i d -
i n g a c a s u al e n v i r o m n e n t i n t e g r a t i n g
v i r t u a l a n d r e a l w o r l d s [ 5 ] .
A t h i r d n e w a p p r o a c h i s t h e u s e o f
c o l l a b o r a t i o n t o s p e c i | } ' i n t o r m a t i o n
f i h e r i n g . D e s c r ib e d i n t h e D e c e m b e r
1992 i s sue o f Communications of the
A C M , t h i s w o r k b y D o u g " I ~e r ry e x -
t e n d s p r e v i o u s n o t i o n s o f i n t o r m a -
t i o n f i l t e r s b y p e r m i t t i n g f i l t e r s t o
r e f e r e n c e o t h e r f i l t e rs , o r t o d e p e n d
o n t h e v a l u e s o f m u h i p l e m e s s ag e s .
F o r i n s t a n c e , o n e c a n s e l e c t a l l m e s -
s a g e s t h a t h a v e b e e n r e p l i e d t o b yS m i t h ( t h e s e m e s s a g e s d o n o t e v e n
m e n t i o n S m i t h , o f c o ur s e ), o r a ll
m e s s a g e s t h a t t h r e e o t h e r p e o p l e
f o u n d i n t e r e s ti n g . I m p l e m e n t i n g t h is
r e q u i r e d i n v e n t i n g t h e i d ea o f a
" c o n t i n u o u s q u e r y , " w h i c h c a n e f fe c -
t i ve l y s a m p l e a c h a n g i n g d a t a b a s e a t
a l l p o i n t s i n t i m e . C a l l e d " T a p e s t r y , "
t h i s s y s t e m p r o v i d e s n e w w a y s f i n
p e o p l e t o i n v i s i b l y c o l l a b o r a t e .
Privacy of LocationC e l l u l a r s y s t e m s i n h e r e n t l y n e e d to
k n o w t h e l o c a t i o n o f d e v i c e s a n d
t h e i r u s e i n o r d e r t o p r o p e r l y r o u t ei n t b r m a t i o n . F o r i n s t a n c e , t h e t r av e l -
i n g p a t t e r n o f a f r e q u e n t c e l l u l a r
p h o n e u s er c a n b e d e d u c e d f r o m t h e
r o a m i n g d a t a o f c e l l u l a r s er v ic e p r o -
v i d e r s . T h i s p r o b l e m c o u l d b e m u c h
w o r s e i n u b i q u i t o u s c o m p u t i n g w i th
i t s m o r e e x t e n s i v e u s e o f c e l l u l a r
w i r e l e ss . So a k e y p r o b l e m w i t h u b i q -
u i t o u s c o m p u t i n g i s p r e s e r v i n g p r i -
v a c y o f l o c a t i o n . O n e s o l u t i o n , a c e n -
t r a l d a t a b a s e o f l o c a t i o n i n f o r m a t i o n ,
m e a n s t h e p r i v a c y c o n t r o l s c a n b e
c e n t r a l i z e d a n d p e r h a p s d o n e w e l l - -
o n t h e o t h e r h a n d o n e b r e a k - i n th e r e
r e v e a l s a l l , a n d c e n t r a l i t y i s u n l i k e l y
t o s c a l e w o r l d w i d e . A s e c o n d s o u r c e
o f i n s e c u r i t y i s t h e t r a n s m i s s i o n o f
t h e l o c a t i o n i n f o r m a t i o n t o a c e n t r a l
s i t e. T h i s s i t e is t h e o b v i o u s p l a c e t o
t r y t o s n o o p p a c k e t s , o r e v e n t o u s e
t r a f f i c a n a l y s i s o n s o u r c e a d d r e s s e s .O u r i n i t i a l d e s i g n s w e r e a l l c e n t r a l ,
i n i t ia l l y w i t h u n r e s t r i c t e d a c c e ss ,
g r a d u a l l y m o v i n g t o w a r d i n d i v i d u a l
u s e r s " c o n t r o l l i n g w h o c a n a c c e s s i n -
f o r m a t i o n a l : ~ ) u t t h e m . O u r p r e -
t i ~ rr e d d e s i g n a v o i d s a c e n t r a l r e p o s i -
t o r y , i n s t e a d s t o r i n g i n l o r m a t i o n
a t × ) u t e a c h p e r s o n a t t h a t p e r s o n ' s
P C o r w o r k s ta t i o n . P r o g r a m s t h a t
n e e d t o k n o w a p e r s o n ' s l o c a t i o nm u s t q u e r y t h e P C , a n d p r o c e e d
t h r o u g h w h a t e v e r s e c u r i t y m e a s u r e s
t h e u s e r h a s c h o s e n t o i n s t a l l .
E u r o P A R C u s e s a s y s t e m o f t h i s s o rt .
A c c u m u l a t i n g i n t o r m a t i o n a b o u t
i n d i v i d u a l s o v e r l o n g p e r i o d s i s o n e
o f t h e m o r e u s e f u l t h i n g s to d o , b u t
q u i c k l y r a is e s h a c k l e s . A k e y p r o b l e m
f o r l o c a t i o n i s h o w t o p r o v i d e o c c a -
s i o n a l l o c a t i o n i n l b r m a t i o n t o r c l i -
e n t s w h o n e e d i t , w h i l e s o m e h o w
p r e v e n t i n g t h e r e l i a b l e a c c u m u l a t i o t ~
o f l o n g - t e r m t r e n d s a b o u t a n i n d i v i d -
u a l . S o f a r a t P A R C w e h a v e e x p e r i -
m e n t e d o n l y w i th s h o r t - t e r m a c c u -n t u l a ti o n o f i n f o r m a t i o n t o p r o d u c e
a u t o m a t i c d a i l y d i a r i e s o f a c t i v i t y
[17].
I t is i m p o r t a n t t o r e a l i z e t h e r e c a n
n e v e r b e a p u r e l y t e c h n o l og i c a l s o lu -
t i o n t o p r i v a c y , a n d s o c ia l i ss u e s m u s t
b e c o n s i d e r e d i n t h e ir o w n r i g h t . I n
t h e c o m p u t e r s c i e nc e l a b w e a r e t r y -
i n g t o c o n s t r u c t s y s t e m s t h a t a r e p r i -
v a c y - e n a b l e d , g i v i n g p o w e r t o t h e
i n d i v i d u a l . B u t o n l y s o c i e ty c a n c a u s e
t h e r i g h t s y s t e m t o h e u s e d . T o h e l p
p r e v e n t f u t u r e o p p r e s s i v e e m p l o y e r s
o r g o v e r n m e n t s t ¥ o m t a k i n g t h i s
p o w e r a w a y , w e a r e a ls o e n c o u r a g i n gt h e w i d e d i s s e m i n a t i o n o f i n f o r m a -
t i o n a b o u t l o c a t i o n s y s t e n t s a n d t h e i r
p o t e n t i a l fo r h a r m , W e h a v e c o o p e r -
a t e d w i t h a n m n b e r o f a r t ic l es i n t h e
San Jose M ercury News, t h e Worthington
Post, a n d t h e New York T imes o n t h i s
t o p i c . T h e r e s u l t , w e h o p e , i s t e c h n o -
l o gi c al e n a b l e m e n t c o m b i n e d w i t h a n
i n f o r m e d p o p u l a c e t h a t c a n n o t b e
t r i c k e d i n t h e n a m e o f t e c h n o l o g y .
Computat ional MethodsA n e x a m p l e o f a n e w p r o b l e m i n th e -
o r e t i c a l c o m p u t e r s c i e n c e e m e r g i n g
f r o m u b i q u i t o u s c o m p u t i n g i s o p t i -
m a l c a c h e s h a r i n g . T h i s p r o b l e m
o r i g i n a l l y a r o s e i n d i s c u s s i o n s o f o p -
t i m a l d i s k c a c h e d e s i g n t b r p o r t a b l e
c o m p u t e r a r c h i t e c t u r e s . B a n d w i d t h
t o t h e p o r t a b l e m a c h i n e m a y b e q u i t e
l o w , w h i l e i t s p r o c e s s i n g p o w e r i s re l -
a t i v e l y h i g h . T h i s i n t r o d u c e s a s a
Mobile Compu ting and Communications Review, Volume 3, Num ber 319
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2 0
p o s s i b l e d e s i g n p o i n t t h e c o m p r e s -
s i o n o f p a g e s i n a R A M : c a c h e , r a t h e r
t h a n w r i t i n g t h e m a l l t h e w a y b ac k
o v e r a s l o w l in k . T h e q u e s t i o n a r i se s
o f ~ h e o p t i m a l s t r a t e g y t 0 r p a r t i ti o n -
i n g m e m o r y b e tw e e n c o m p r e s s ed
a n d u n c o m p r e s s e d p a g e s .
T h i s p r o b l e m c a n b e g e n e r a li z e d
a s t o l l o w s [ 1 ] :
" I 'h e C a c h e S h a r i n g P r o b l e m . A
p r o b l e m i n s t a n c e i s g i v e n b y a s e -
q u e n c e o f p a g e r e q u e s ts . P a g e s a r e o f
t w o t y p e s , U a t td C ( f b r u n c o n t -
p r e s s e d a n d c o m p r e s s e d ) , a n d e a c h
p a g e i s e i t h e r I N o r O U T . A r e q u e s t
i s s e r v e d b y c h a n g i n g t h e r e q u e s t e d
p a g e t o I N i f i t i s c u r r e n t l y O U T . h f i -
t ia l ly al l p a g e s a r e O U T . T h e c o s t t o
c h a n g e a t y p e - U ( t y p e - C ) p a g e f r o m
O U T t o I N i s C u ( r e s p e c t i v e l y , C c ) .
W h e n a r e q u e s t e d p a g e i s O U T , w e
s a y t h a t t h e a l g o r i t h n t m i s s e d . R e -
m o v i n g a p a g e f r o m m e m o r y is f r e e .
L o w e r B o u n d T h e o r e m : N o de t er -
m i n i s fi c , o n - l i n e a l g o r i t h m f b r c a c h e
s h a r i n g c a n b e c - c o m p e t i t i v e f o r
c < M A X (1 + C t : / ( C t ~ + C c ) , 1 +
C c i ( C t ; + C c ) )
T h i s l o w e r b o u n d t o r c r a n g e s
f l - o m 1 . 5 t o 2 , a n d n o o n - l i n e a l g o -
r i t h m c a n a p p r o a c h c l o s e r t o th e o p -
t i m u m t h a n t h i s f a c t o r . B e r n e t a l . [ 11
a l s o c o n s t r u c t e d a n a l g o r i t h m t h a t
a c h i e v e s t hi s f a c to r , t h e r e t o r e p r o -
v i d i n g a n u p p e r b o u n d a s w el l. T h e y
f h r t h e r p r o p o s e a s et o f m o r e g e n -e r a l s y m b o li c p r o g r a m m i n g t o ol s f b r
s o l v i n g c o m p e t i t i v e a l g o r i t h m s o f th i s
s o r t .
Concluding R emarksA s w e s t a r t t o p u t t a b s , p a d s , a n d
b o a r d s i n t o u s e , t h e f i r s t p h a s e o f
u b i q u i t o u s c o m p u t i n g s h o u l d e n t e r
i ts m o s t p r o d u c t i v e p e r i o d . W i t h t h i s
s u b s t r a t e i n p l a c e w e c a n m a k e m u c h
m o r e p r o g r e s s b o t h i n e v a l u a t in g o u t
t e c h n o l o g i e s a n d i n c ht x > si n g o u r
n e x t s t e p s . A k e y p a r t o f t h i s e v a l u a -
t i o n i s u s i n g t h e a n a l y s e s o f p s y c h o l o -
g i st s , a n t h r o p o l o g i s t s , a p p l i c a t i o nw r i t e r s , a r t i s t s , m a r k e t e r s , a n d c u s -
t o m e r s . W e b e l i ev e t h e y w i l l f i n d
s o m e l i g a t u r e s w o r k w e l l ; w e k n o w
t h e y w i ll f i n d s o m e f e a t u r e s d o n o t
w o r k . T h u s w e w il l b e g i n a g a i n t h e
c y c l e o f c r o s s - d i s c i p l i n a r y f e r t i l iz a -
t i o n a n d l e a r n i n g . U b i c o m p i s l i ke l y
t o p r o v i d e a f r a n t e w o r k f o r i n te r e s t -
i n g a nd p r o d u c t i v e w o r k t b r m a n y
m o r e y e a r s o r d e c a d e s , b u t w e h a v e
m u c h t o l e a r n a b o u t t h e d e t a i ls .
A c k n o w l e d g m e n t s
T h i s w o r k w a s f u n d e d b y X e r o x
P A R C . P o r t i o n s o f " t h i s w o r k w e r e
s p o n s o re d u n d e r c o n t r ac t # D A B q
6 3 - 9 1 - 0 0 2 7 . U b i q u i t o u s c o m p u t i n g iso n l y a sm a l l p a r t o f t h e w o r k g o i n g
o n a t P A R C , a n d w e a r e g r a t e f t f l t b )
P A R C ' s r i c h , c o o p e r a t i v e , a n d f e r t i l e
e n v i r o m n e n t i n s u p p o r t o f th e d o c u -
m e n t c o m p a n y . []
R e f e r e n c e s
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t o r s: C . 3 [ C o m p u t e r S y s t e m s O r g a n i z a -
t i o n] : S p e c i a l - p u r p o s e a n d a p p l i c a t i o n -
b as ed s y s t em s ; 1 " t . l . 2 [ In fo rm a t i o n S y s -
t e m s l : M o d e l s a n d P r i n c i p l e s - -
User~Machine . ~y st e,~' ; H A . I [ In fo rm a t i o n
S y s t e m s ] : I n f i ) r m a t i o n S y s t e m s A p p l i c a -
t i o n s - - O f f i c e Autmnatirm: J . 0 [ C o m p u t e r
A p p l i c a t i o n s ] : G e n e r a l
G e n e r a l T e r m s : D e s i g n, t l u m a n F a c-
t o r s
A d d i t i o n a l K e y W o r d s a n d P h r a s e s ;
U b i q u i to u s c o m p u t i n g
A b o u t t h e A u t h o r :
M A R K W E I S E R i s p r i n c i p a l ~ 'i c n t i st a n d
m a n a g e r o f t h e C o m p u t e r S c i en c e la b o r a-
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l e c t io n ) , v i s u a li z a t io n o f o p e r a t i n g s y s t e m
i n t e r n a l s , u b i q u i t o u s c o m p u t i n g a n d
e m b o d i e d v i r t u a l i t y . A u t h o r ' s P r e s e n t
A d d r e s s : X e r o x P A R C , 33 .3 3 C o y o t e H i ll
R o ad , P a l o A l t o , C A 9 4 3 0 4 ; e m a i h w e i s e r.
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c o n t r o l [ 1 l, B y i n t e r c o n n e c t i n g P A R C 's
r ic h c o m p u t a t i o n a l i n f r a s t ru c t u r e w i t h a
c o m p u t e r i z e d b u i l d i n g m a n a g e m e n t s y s-
t e m ( BM S ) t h a t c o n t r o l s h e a t i n g , a ir c o n -
d i t i o n i n g , l i g h t i n g a n d d e s l d : o p a p p l l -
a n c e s , w e p l a n to e x p l o r e n e w s t r a te g i e s
f o r e n e r g y c o n s e r v a t io n a n d o f f i ce c o m -
f o r t c o n t r o l .
O v e r t h e p a s t 2 0 y e a r s , t h e d e s i g n e r s
o f h e a t i n g , v e n t i la t i o n , a n d a i r c o n d i t i o n -
i n g ( HV A C ) s y s t e m s h a v e g r a d u a l l y
s h i f t e d t o w a r d t h e u s e o f d i g it a l c o m -
p u t e r s [ 2 ] , r e p l a c i n g d i r e c t m a n u a l c o n .
t r o l a n d s i m p l e a n a l o g f e e d b a c k l o o p s
s u c h a s th e r m o s t a t s . D i g i t a l c o n t r o lm a k e s p o s s i b l e m o r e f l e x i b l e , p r e c i s e ,
a n d c o m p l e x c o n t r o l s t r a t e g i e s t h a t I n
t u r n c a n p r o v i d e s i g n i f i c a n t e n e r g y s a v-
i n gs . F o r e x a m p l e , m o s t c o m p u t e r -
c o n t r o l l e d b u i ld i n g s o f f e r a u to m a t i c
t e m p e r a t u re s e t b a c ks t o r e d u c e e n e r g y
c o n s u m p t i o n a f t e r w o r k i n g h o u r s an d o n
w e e k e n d s . M a n y s y s t e m s a ls o c o n t r o l
l i g h t in g t o s a v e a d d i t i o n a l e n e r g y .
C u r r e n t p r o g r e s s i n l o w - c o s t d i s t ri b -
u t e d c o m p u t in g , c o m m u n i c a ti o n s , a n d
s e n s i n g t e c h n o l o g i e s w i l l e n a b l e f u t u r e
b u il d i ng m a n a g e m e n t s y st e m s t o b e
m u c h m o r e r e s p o n s i v e t o i n d iv i d u a l
p r e f e r e n c e s a n d a c t iv i ti e s . T h e f o l l o w i n g
a r e e x a m p l e s o f t h e t y p e s o f e n h a n c e -m e n t s t h a t a r e p o s s i b l e :
• S m a l l. l o c a t i o n - s e n s i n g m o b i l e c o m p u t .
e r s s u c h a s P A RC T ab s p r o v i d e a n i d e a l
i n t e r f a c e t o a l l o w o f f ic e o c c u p a n t s a n d
m a i n t e n a n c e s t a f f t o s e t p a r a m e t e r s
i s u c h a s pr e f e r r e d r a n g e s f o r t e m p e r a -
t u r e a n d l i g h t l e v e l) a n d r e c e i v e f e e d -
b a c k a b OU t c u r r e n t c o n d i t i o n s .
• O c c u p a n c y s e n s o r s a r e a l r e a d y U S e d a t
P AR C t o a v o i d h e a t i n g o r c o o l i n g c o n f e r -
e n c e r o o m s w h e n t h e y a r e n o t i n us e .
W e a re n o w e x p e r i m e n t i n g w i t h u s e r -
s e l e c t a b le s t r a t e g i e s f o r s w i t c h in g o f f
l i g h t s , c o m p u t e r d i s p l a y s , a n d o t h e r a p -
p l ia n c e s a n d f o r s e t t i n g b a c k t h e a i r c o n -
d i t i o n i n g w h e n o f f i c e s a r e u n o c c u p i e d .• o t h e r s e n s o r s c a n b e i n te g r a t e d i n t o
t h e s y s t e m t o p r o v i d e a d d i t i o n a l a u t o -
m a t i c f u n c t i o n a l i t y , s u c h a s a d j u s t i n g a r-
t i fi c i a l l i g h t s a n d / o r c l o s i n g b l in d s t o
c o m p e n s a t e f o r c h a n g i n g n a t u r a l l ig h t
l e ve l s, l o w e r i n g l i g h t l e v e l s w h i l e a w o r k -
s t a t i o n i s I n u s e , a n d a d j u s t i n g v e n t i l a -
t i o n d e p e n d i n g o n w h e t h e r d o o r s a re
o p e n e d o r c l o s e d .
• A c t iv e b a d g e s a n d o n - l i n e c a l e n d a r s
m a y b e u s e d t o i d e n t i f y pe r i o d s i n w h i c h
a n o f f ic e ' s o c c u p a n t w i l l b e a w a y f o r a ne x t e n d e d p e r i o d o f t im e ( e .g ., b e c a u s e
t h e o c c u p a n t IS a t a m e e t i n g , a t lu n c h ,
o r a w a y f r o m t h e b u il d in g ) .
TO q u a n t i f y t h e p o t e n t i a l f o r e n e r g y
s a v i n g s b y s u c h s t r a t e g i e s , w e c o n -
d u c t e d a d e t a il e d o n e - d a y e n e r g y a n d
o c c u p a n c y a u d i t o f P A R C 's C o m p u t e r S c i -
e n c e L a b o r a t o ry . T his s t u d y r e v e a l e d t h a t
o f f i c e s a r e t y p i c a l l y v a c a n t f o r 5 0% o f t h e
a v e r a g e 9 - h o u r w o r k d a y , I n i t i a l c a lc u l a -
t i o n s s u g g e s t t h a t o c c u p a n c y - ba s e d c o n -
t r o l o f l i gh t s a n d c o m p u t e r m o n i t o r s
a l o n e w o u l d s a v e $ 4 5 ,0 00 p e r y e a r a t
P AR C, a n d s i m i l a r c o n t r o l o f t h e a i r c o n -
d i t i o n i n g s y s t e m c o u l d s a v e as m u c h a s
$ 9 0 , 0 0 0 .E n c o u r a g e d b y t h e s e e s t im a t e s , w e
h a v e C O n S tr uc te d a f le x i b l e h a r d w a r e a n d
s o f t w a re t e s t b e d t o e x p e r i m e n t w i t h
n e w e n e r g y c o n t r o l s t r a t e g ie s . T h e
t e s t b e d c o n s i s ts o f 1 3 o f f i c e s a n d s o m e
a d j a c e n t p u b li c a r e a s . W i t h i n e a c h o f f i c e
w e h a v e i n s t a l le d t e m p e r a t u r e , l i g h t
l e v e l, o c c u p a n c y , a n d a c t i v e b a d g e s e n -
s o rs to g e t h e r w i t h c o m p u t e r - c o n t r o ll e d
v e n t i l a t i o n , h e a t i n g , e l e c t r i c o u t l e t s , a n d
o v e r h e a d l i g h t in g . T h e s e d e v i c e s a r e
c o n n e c t e d t o a c o n v e n t io n a l c o m p u t e r -
i z e d b u i l d i n g m a n a g e m e n t s y s t e m .
w h i c h i n t u r n i s c o n n e c t e d t o t h e P AR C
c o m p u t e r n e t w o r k t h r o u g h a g at e w a y,
u s i n g a c o n v e n t i o n a l b u il d i n g m a n a g e -m e n t s y s t e m a s t h e b a c k b o n e a ll o w s u s
t o e n s u r e t h a t t h e s y s t e m ' s ba s ic f u n c -
t i o n a l i t y i s h i g h l y r e l i a b le , w h i l e s t i l l a b
l o w i n g n e w c o n t r o l s t r a te g i e s a n d d a t a
a na ly s is t o b e r u n f r o m w o r k s t a t i o n s .
AS O f t h i s w r i t i n g ( M a r c h) t h e c o m p l e t e
s y s t e m i s J us t c o m i n g o n d l n e a n d w e a r e
b e g i n n i n g t o c o l l e c t t h e b a s e l in e d a t a
n e e d e d t o e v a l u at e f u t ur e e n e r g y s a v-
i n g s . A t p r e s e n t , t h e s y s t e m c o n t i n u -
o u s l y l o g s o v e r 8 0 0 v a ri a bl e s fr o m t h e
t e s t a r e a a n d f r o m o t h e r p a rt s o f t h e
b u il d i n g . O v e r t h e n e x t f e w m o n t h s w e
p l an t o e x p e r i m e n t w i t h a v a r ie t y o f c o n *
t r o l s t r a t e g i e s a n d u s e r i n t e r f a c e s ( UI s )
a n d m e a s u r e t l l e l r e f f e c t s .
In t h e d e v e l o p m e n t o f th i s s y s te m w e
e n c o u n t e r e d a n u m b e r o f d e s i g n is s ue s
c o m m o n t o m a n y u b i qu i to u s c o m p u t i n g
s y s t e m s :
• r e l i a b i l i t y a n d i n v i s i b i l i ty : C o m p u t e r s y s -
t e m s e n t r u s t e d w i t h s u c h b a s ic a s p e c t s
o f c o m f o r t a s t e m p e r a t u r e a n d l i g h t l e v-
e l s m u s t o b v i o u s l y b e h i g h l y r e l ia b le .
J~d? 1993d~kfl~(i. No.)~ l~(l~¢J(lult),ll(~i~lr)l)ll,Is F YllQllA~M
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